LENOX, Mass — "Old friends are acting strange, they shake their heads, they say I've changed." says Joni Mitchell in her song "Clouds." — "but something's lost and something's gained."

Things have indeed changed a lot since she first began to perform publicly. The halls are larger now, the crowds are not so attentive, and her music has begun to take on the style of rock or jazz more than ever.

A STANDARD

Yet she remains a standard in popular music. Her capacities as a writer of poetry, set to music, are unsurpassed in the pop field. Many people lament her passage into pop, since she no longer turns out gently introspective lyrics like "The Urge for Going" or "The Circle Game." But her description of the pressures and satisfaction found in the music world are clearly the most articulate, in a field flooded with such material.

Tom Scott and the LA Express provide her well-executed musical back-up, and their fusion of different styles with her music is done extremely well. Despite this it is hard to say what if anything has been gained.

Five or six years ago she was writing songs about a life which was much like an Everyman's, about her friends, family, and feelings. But as her experiences changed her subject matter did too.

Now she sings, in "Free Man in Pari," that she would "go back there tommorrow, but for the work I've taken on, stoking the star-making machinery behind the popular song."

EARLIER DAYS

In earlier days she could be seen in small comfortable halls, performing her material alone accompanied only by her delicate, masterful guitar or still unsure piano. That experience was like visiting a good friend. She made her audience feel they were in her living room. She would talk to them, and they would hang on every word or note.

Transforming Tanglewood's shed, which holds 6,000 persons, into an intimate performance hall is virtually impossible, although some of the earlier flavor still comes through as she asks the crowd questions about their lives. Unfortunately she still seems slightly unsure of what to before all those people, and response at a concert of that size consists of a few persons yelling whatever comes into their heads.

While she has adopted the format popular in many rock concerts of sandwiching her softer, more sensitive, solo material between two slices of electrified sound, it is the middle portion most people come to hear, and that section, reminiscent of her early days, is most successful.

Sti11 the show works well. She achieves a reasonably close contact with her audience, and her material always communicates a genuine sensitivity.

In her performance there were old songs and new songs, most of them from her latest album, "Court and Spark." But she did reach back and treat people to songs from earlier albums, like "Clouds," as well as including some material as yet unreleased.

During her talk about what it is like being a "star" she said that the poet and the performer are always in conflict. Joni Mitchell represents a performer who has made compromises to the public demand. She has become increasingly a reflection of the popular image of what she should be, yet the artistry still dominates.